Such an arrangement is frequently used for head-up displays (HUDs) and head/helmet-mounted displays (HMDs) in products such as vehicle windscreen displays, military helmets, virtual reality (VR) headsets, augmented reality (AR) headsets and ‘smart glasses’. In such applications, positioning the optical projection system away from the viewing axis means that the forward view need not be obstructed, allowing for transparent displays. It also means that the display system can be arranged in a compact manner, for example, by locating the optical projection system to the side of the head in a HMD application.
Conventionally, the primary reflecting optic may be either a beam combiner that is partially reflecting and partially transmitting, or it may be a purely reflecting component. Examples of how the primary reflecting optic may be implemented include the following:    1. a surface of the transparent window already existent in the device (e.g. a windscreen);    2. a partially reflecting structure incorporated in a transparent window;    3. an additional transparent component, for example a flat plate of glass acting as a beam combiner in traditional HUD devices;    4. a non-transparent component in a VR device.
Traditional HUD devices use a flat or nearly flat reflector angled at around 45 degrees from the viewing axis, as in 1 or 3 above. This permits viewing of the reflection of a large conventional display image source (such as an LCD display panel) located close to the reflector. This approach is simple but it has some disadvantages. Without magnification in the reflector, the source display must be as large as the desired virtual image, making the display and related optics very bulky. Furthermore, it is not possible to control the apparent distance of the virtual image, which is particularly problematic for near-to-eye applications, where the image distance must be extended significantly to give a comfortable viewing distance.
Some traditional HUDs use additional optics to control distortion and virtual image distance. This improves image quality and position, but adds considerable bulk and does not reduce the requirement for the output window of the projection optics to be of similar angular size to the displayed image.
In contrast, other approaches use a reflector that is significantly non-planar and therefore provides significant optical power to the imaging system. This allows a much smaller source image display (e.g. one of various types of ‘microdisplay’) to be magnified and presented to the viewer at a controlled virtual image distance. A necessary compromise when applying magnification is that the volume of space in which the displayed image is viewable (the ‘eyebox’) is restricted, but this is often acceptable when the position of the viewing optic is controlled sufficiently accurately.
The form of the reflector in these systems is typically an ‘off-axis’ paraboloid, biconic or similar shape. Such a reflector form can be relatively bulky however, and the unattractive and bulky appearance of such designs limits the applications.
The reflector can be made more visually appealing and compact by segmenting the reflecting surface as a ‘grating’ structure in a similar manner to a Fresnel lens. This allows the reflection angle from the surface to be controlled independently from the base curvature of the substrate in which the grating is formed. Such a grating structure can be optionally embedded within a transparent material as disclosed in WO 2011124897 A1. By additionally making the reflector partially transmissive, this allows the transmitted portion of the light to be undeviated so that the view through the embedding optic is substantially unaffected. Such an approach allows the implementation of HMDs and HUDs with beam combiners embedded within arbitrarily shaped transparent components such as spectacle lenses, helmet visors and windscreens.
Reflectors with magnifying power can provide design flexibility and enable the use of microdisplays. However, if used to directly image a display source, they give very poor image quality because the reflecting surface introduces various optical aberrations. To recover a good image, additional optical components are required to counteract the aberrations caused by the reflector.
The primary reflecting optic can be designed to be a freeform asphere or freeform asphere grating, giving some flexibility in controlling aberrations, but a single surface alone cannot provide sufficient degrees of freedom to prevent aberrations for an extended image. For an axially symmetric system, the reflector would be expected to introduce spherical aberration, coma, distortion, astigmatism, tilt and other higher order aberrations. This system requires a strongly off-axis reflection, so the surface also introduces more complex binodal aberrations.
In the case of an embedded grating structure, embedding the segmented reflector within a transparent substrate which may have a curved surface introduces additional aberrations. Typically the embedding component follows a curve that is concave on the side oriented toward the viewing optics and projection optics. In this case the light rays encounter a negative powered refractive interface between the reflector and viewing optics, and a highly tilted negative powered refractive interface between the reflector and projecting optics. These surfaces contribute further to the aberrations described above, but also add colour dependence to the aberrations. In particular, the tilt introduces strong lateral colour separation.
Systems of optics designed to counteract these aberrations often suffer from other disadvantages, including: high complexity or large number of optical components leading to high component cost, and tight assembly tolerances, leading to high assembly costs; a large or undesirable space envelope, making it difficult to build the optics into a constrained space, and constraints on position, shape and orientation of the primary reflecting optic.